In preparation for removing some of the MIPS compiler intrinsics from
arch/mips/lib, first update the build of vmlinuz to use the generic
ashldi3 from lib.

Both ashldi3 and bswapsi objects need to be built with different CFLAGS
for inclusion to vmlinuz rather than simply including the object built
for the main kernel image. The objects cannot be built directly from
source, since CONFIG_MODVERSIONS changes cmd_cc_o_c to prevent this.

Split the rule to ship ashldi3 and bswapsi from the relevant source
locations.

These files make no reference to other files in their directory, so the
additional CFLAGS are apparently unnecessary - remove them as well.
Signed-off-by: Matt Redfearn <matt.redfearn@mips.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Antony Pavlov <antonynpavlov@gmail.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/19050/
[jhogan@kernel.org: Add if_changed and FORCE to fix build failure when
 arch/mips/boot/compressed/ashldi3.c is already generated but there is
 no .ashldi3.c.cmd file yet]
Signed-off-by: James Hogan <jhogan@kernel.org>